Arthur John Johnson was born in Galveston, Texas, on March 31, 1878, the year after the collapse of Reconstruction. His parents were ex-slaves  his father, Henry, was a school janitor, and his mother, Tina (known as "Tiny"), was a laundress. Arthur, who later reversed the order of his given names and took the nickname Jack, was the third of nine children, five of whom reached adulthood. Henry and Tiny made certain that all of their children received at least five years of schooling, and the couple managed to buy their own home at 808 Broadway, on the eastern end of the island in Galveston's racially-mixed Twelfth Ward.

As a boy, Jack Johnson was a part of the racially mixed 11th Street and Avenue K gang, although he claimed in his 1927 autobiography that he avoided fighting until he was 12 years old. While a black high school was available to Johnson, he did not attend. Instead, he went to work to help support his family. He swept out a barbershop and worked as a porter in a gambling parlor and as a baker's assistant. At one point, he hoboed to Dallas to find work and apprenticed to a man who painted carriages. The shop owner, Walter Lewis, introduced Johnson to boxing.

Johnson claimed to have left home again when he was 16 years old. He said he traveled all the way to Manhattan to shake the hand of Steve Brodie, an Irish immigrant who claimed to have survived a leap in 1886 from the then-new Brooklyn Bridge. Johnson allegedly continued on to Boston to meet another of his idols, a welterweight from Barbados named Joe Walcott. Johnson returned to Galveston and worked as a janitor, a stableboy, a hotel porter and a longshoreman. At one point, he got a job cleaning a gym and saved up enough money to buy a pair of boxing gloves. His first real fight was on the beach in the summer of 1895, against another dockworker named John Lee. Johnson won the fight and the $1.50 purse. Later that summer, professional boxer Bob Thompson came to Galveston, offering $25 to anyone who could survive four rounds in the ring with him. Johnson made it through four rounds, but called the $25 "the hardest earned money of my life."

According to his autobiography, Johnson married a childhood friend named Mary Austin in 1898. No record of the marriage exists, and the 1900 census shows Johnson still living at home with his parents and siblings. Whatever his marital state, Johnson soon realized that the options in his hometown were limited.

There was nothing more for me to do in Galveston. The purses offered me were truly minimal  10, 15 or 20 dollars at most. If I stayed there, all I'd have is debts for I had to pay one or two seconds and their wages absorbed the whole purse and sometimes more. So I decided to travel the world, to try to box from one coast to the other, and to attach myself to the training camp of a famous boxer.